Microfabricated air bridges for planar microwave resonator circuits

ABSTRACT

The present invention provides a process and structure of microfabricated air bridges for planar microwave resonator circuits. In an embodiment, the invention includes depositing a superconducting film on a surface of a base material, where the superconducting film is formed with a compressive stress, where the compressive stress is higher than a critical buckling stress of a defined structure, etching an exposed area of the superconducting film, thereby creating the at least one bridge, etching the base material, thereby forming a gap between the at least one bridge and the base material, depositing the at least one metal line on at least part of the superconducting film and at least part of the base material, where the at least one metal line runs under the bridge.

BACKGROUND

The present disclosure relates to integrated circuit chips, and morespecifically, to microfabricated air bridges for planar microwaveresonator circuits.

SUMMARY

The present invention provides a process and structure ofmicrofabricated air bridges for planar microwave resonator circuits. Inan embodiment, the process includes depositing a superconducting film ona surface of a base material, where the superconducting film is formedwith a compressive stress, where the compressive stress is higher than acritical buckling stress of a defined structure, forming a firstphotoresist pattern on the superconducting film, where the photoresistpattern defines at least one bridge etching an exposed area of thesuperconducting film, thereby creating the at least one bridge, etchingthe base material, thereby forming a gap between the at least one bridgeand the base material, forming a second photoresist pattern on at leasta defined surface of the base material, where the second photoresistpattern defines a space for at least one metal line, cleaning at least apart of the surface of the superconducting film, depositing the at leastone metal line on at least part of the superconducting film and at leastpart of the base material, where the at least one metal line connects afirst part of the superconducting film and a second part of thesuperconducting film, where the at least one metal line runs under thebridge, and removing the second photoresist pattern.

In an alternative embodiment, the process includes depositing a firstsuperconducting film on a surface of a base material, depositing asecond superconducting film on a surface of the first superconductingfilm, where the second superconducting film is formed with a compressivestress, where the compressive stress is higher than a critical bucklingstress of a defined structure, forming a first photoresist pattern onthe second superconducting film, where the photoresist pattern definesat least one bridge, etching an exposed area of the secondsuperconducting film, thereby creating the at least one bridge, etchingthe first superconducting film, thereby forming a gap between the atleast one bridge and the base material, forming a second photoresistpattern on at least a defined surface of the base material, where thesecond photoresist pattern defines a space for at least one metal line,cleaning at least a part of a surface of the second superconductingfilm, depositing the at least one metal line on at least part of thesecond superconducting film and at least part of the base material,where the at least one metal line connects a first part of the secondsuperconducting film and a second part of the second superconductingfilm, where the at least one metal line runs under the bridge, andremoving the second photoresist pattern.

In an embodiment, the structure includes a base material, a first layerof a metal formed on the base material with a compressive stress, andone or more layers of the metal formed on the first layer, where the oneor more layers has a compressive stress different than the compressivestress of the first layer. In an embodiment, the first layer and the oneor more layers comprise a superconducting material.

In an alternative embodiment, the structure includes a base material, athin film on the base material, an area between a first portion of thethin film and a second portion of the thin film where the thin film ispartially removed, and a thin film bridge between the first portion andthe second portion, where the bridge, the first portion, and the secondportion comprise a monolithic structure. In an embodiment, the structurefurther includes a third portion of the thin film between the firstportion and the second portion and not connected to the first portionand the second portion, a fourth portion of the thin film between thefirst portion and the second portion and not connected to the firstportion and the second portion, and a metal line running under the thinfilm bridge and connecting the third portion to the second portion.

In an embodiment, the structure includes a first metal area on a basematerial, a second metal area on the base material, a bridge connectingthe first metal area and the second area, where the first metal area,the second metal area, and the bridge comprise a monolithic structure, afirst metal line between the first metal area on the base material andthe second metal area on the base material, a second metal line betweenthe first metal area on the base material and the second metal area onthe base material, and a third metal line between the first metal areaon the base material and the second metal area on the base material,where the third metal line connects the first metal line and the secondmetal line. In further embodiment, the first metal line and the secondmetal line comprise a same material, and the third metal line comprisesa material different from the material of the first metal line and thesecond metal line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 1B depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 1C depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 1D depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 1E depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 1F depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 1G depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 1H depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 1I depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2A depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2B depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2C depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2D depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2E depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2F depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2G depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2H depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 2I depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 3A depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 3B depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 4A depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 4B depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 4C depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 5 depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 6 depicts a diagram in accordance with an exemplary embodiment ofthe present invention.

FIG. 7 depicts a flow diagram in accordance with an exemplary embodimentof the present invention.

FIG. 8 depicts a flow diagram in accordance with an exemplary embodimentof the present invention.

DETAILED DESCRIPTION

The present invention provides a process and structure ofmicrofabricated air bridges for planar microwave resonator circuits. Inan embodiment, the process includes depositing a superconducting film ona surface of a base material, where the superconducting film is formedwith a compressive stress, where the compressive stress is higher than acritical buckling stress of a defined structure, forming a firstphotoresist pattern on the superconducting film, where the photoresistpattern defines at least one bridge etching an exposed area of thesuperconducting film, thereby creating the at least one bridge, etchingthe base material, thereby forming a gap between the at least one bridgeand the base material, forming a second photoresist pattern on at leasta defined surface of the base material, where the second photoresistpattern defines a space for at least one metal line, cleaning at least apart of the surface of the superconducting film, depositing the at leastone metal line on at least part of the superconducting film and at leastpart of the base material, where the at least one metal line connects afirst part of the superconducting film and a second part of thesuperconducting film, where the at least one metal line runs under thebridge, and removing the second photoresist pattern.

In an alternative embodiment, the process includes depositing a firstsuperconducting film on a surface of a base material, depositing asecond superconducting film on a surface of the first superconductingfilm, where the second superconducting film is formed with a compressivestress, where the compressive stress is higher than a critical bucklingstress of a defined structure, forming a first photoresist pattern onthe second superconducting film, where the photoresist pattern definesat least one bridge, etching an exposed area of the secondsuperconducting film, thereby creating the at least one bridge, etchingthe first superconducting film, thereby forming a gap between the atleast one bridge and the base material, forming a second photoresistpattern on at least a defined surface of the base material, where thesecond photoresist pattern defines a space for at least one metal line,cleaning at least a part of a surface of the second superconductingfilm, depositing the at least one metal line on at least part of thesecond superconducting film and at least part of the base material,where the at least one metal line connects a first part of the secondsuperconducting film and a second part of the second superconductingfilm, where the at least one metal line runs under the bridge, andremoving the second photoresist pattern.

In an embodiment, the structure includes a base material, a first layerof a metal formed on the base material with a compressive stress, andone or more layers of the metal formed on the first layer, where the oneor more layers has a compressive stress different than the compressivestress of the first layer. In an embodiment, the first layer and the oneor more layers comprise a superconducting material.

In an alternative embodiment, the structure includes a base material, athin film on the base material, an area between a first portion of thethin film and a second portion of the thin film where the thin film ispartially removed, and a thin film bridge between the first portion andthe second portion, where the bridge, the first portion, and the secondportion comprise a monolithic structure. In an embodiment, the structurefurther includes a third portion of the thin film between the firstportion and the second portion and not connected to the first portionand the second portion, a fourth portion of the thin film between thefirst portion and the second portion and not connected to the firstportion and the second portion, and a metal line running under the thinfilm bridge and connecting the third portion to the second portion.

In an embodiment, the structure includes a first metal area on a basematerial, a second metal area on the base material, a bridge connectingthe first metal area and the second area, where the first metal area,the second metal area, and the bridge comprise a monolithic structure, afirst metal line between the first metal area on the base material andthe second metal area on the base material, a second metal line betweenthe first metal area on the base material and the second metal area onthe base material, and a third metal line between the first metal areaon the base material and the second metal area on the base material,where the third metal line connects the first metal line and the secondmetal line. In further embodiment, the first metal line and the secondmetal line comprise a same material, and the third metal line comprisesa material different from the material of the first metal line and thesecond metal line.

Superconducting coplanar waveguide (CPW) transmission lines andresonators are integral components of cryogenic detectors forsubmillimeter electromagnetic radiation, quantum memory elements, andsolid-state quantum computing architectures. The desired mode profile ofa CPW is symmetric, with the two ground planes on either side of thecenter trace held to the same electrostatic potential. However,asymmetries and discontinuities in the microwave circuitry can lead tothe excitation of spurious slot line modes. These modes can couple toelements of the circuit such as qubits, and they represent a source ofloss and decoherence.

Crossover connections between the ground planes that are interrupted bythe CPW structure can suppress these spurious modes. Free standingcrossovers, known as airbridges, have been a staple of conventionalmicrowave CPW technology and fabrication processes have recently beendeveloped for building airbridges on superconducting microwave circuits.Some methods for fabricating airbridges add processing steps that maydegrade the quality of the circuit. In addition, care must be taken inorder to avoid accidentally creating tunnel junctions with smallcritical currents at the interfaces of such structures. In someembodiments, bridges are formed by an additive process of depositing ametal bridge on a film. In an embodiment, the depositing is performed byevaporation. In an embodiment, depositing the at least one metal line isperformed by evaporation. This process can lead to weakness in theinterface between the metal bridge and the metal film.

In some instances, as the number of qubits in the quantum processorincreases, qubits become topologically isolated and hence they becomefenced in. Such isolated qubits can be reached by using airbridges atthe intersections of signal carrying CPW lines, carrying signal of oneline hoping over another. In an embodiment of the present disclosure,bridges carrying a signal are used to hop over another signal carryingline, along with the ground lines. In such a case, the air gap betweenthe center signal carrying bridge (suspended) line and correspondingground lines has to be adjusted to match the impedance requirements.

In an embodiment, a patterned thin film on a base material including abridge, a broken line running substantially orthogonal to the bridgestructure and interrupted near the bridge structure, a first groundplane on the first side of the broken line and connected to a first endof the bridge, and a second ground plane on a second side of the brokenline connected to a second end of the bridge. In an embodiment, thebridge, the first ground plane, and the second ground plane are amonolithic structure. For example, this monolithic structure couldresult in a much stronger bridge since there is no bridge/film interfaceas in other air bridge designs. In an embodiment, a second metal isdeposited to form an underpass underneath the suspended structure. Thesecond metal is deposited partially on the exposed base material andpartially on the thin film of the broken line, thereby connecting twoportions of the broken line.

Stress Gradients in Thin Films

In an embodiment of the present invention, thin films are produced withinduced stress gradients. In an embodiment, a compressively stressedfilm is deposited with stress higher than a critical buckling stress ofthe designed bridge with a stress gradient across the thickness thatresults in directional buckling (away from the base material). In anembodiment, the thin films are deposited in multiple different layerswith varying induced stresses in each layer. In an embodiment, theaverage stress gradient across the thickness of the bridge is higherthan the critical buckling stress of the suspended structure in order tofacilitate buckling. In an embodiment, the films are niobium, niobiumnitride, titanium nitride, and/or niobium titanium nitride. In anembodiment, the superconducting film comprises a material selected fromthe group consisting of aluminum and niobium.

In an embodiment, the films are any superconducting film. In anembodiment, the thin films are sputtered on. In an embodiment, the argoncontent of the sputtering chamber is varied to change the compressivestress in the thin film. In an embodiment, each of the layers of thethin film range from 5 nm to 50 nm. In an embodiment, the thin filmthickness ranges from 50 nm to 500 nm thick. In an embodiment, a firstsacrificial thin film will be formed without a stress gradient and asecond thin film will be formed with a stress gradient.

Buckling

In an embodiment, the average stress across the thickness of the thinfilm is higher than a critical buckling stress of the designed structurethat will be suspended. The compressive stress in the film will causethe designed structure to buckle. In an embodiment, the criticalbuckling stress of a thin film patterned into a beam (i.e., a bridge) isgiven byσ_(c)=π² EL ²/3t ²  Equation #1where E, L and t are the young's modulus, length and thickness of thebeam respectively. Maximum deflection (V_(max)) due to a compressivestress σ>σ_(c) is given byV _(max)=2((σ/E)(L/π)² −t ²/3)^(1/2)  Equation #2

In an embodiment, the compressive stress, the length, and the thicknessof the beam are adjusted to get the desired deflection. For example,length of the bridge structures can be increased without effecting theCPW boundaries by making an underpass for the ground along with thecentral signal carrying line. For example, it could be necessary to makethe width of the bridge structure larger than the thickness, to avoidlateral buckling of the beam. Once the buckling is limited to thevertical direction, preferential upward buckling, as opposed to downwardbuckling, could be achieved by having a stress gradient across thethickness of the film, as long as the average compressive stress acrossthe thickness is higher than critical buckling stress, as required toachieve buckling. In an embodiment, the film is formed of multiplelayers, and a compressive stress of each layer is different than thecompressive stress of a previous layer. In an embodiment, a compressivestress of each of the one or more layers is different than a compressivestress of a previous layer among the first layer and the one or morelayers. In an embodiment, the layer is a superconducting material.

First Photoresist Layer

In an embodiment, photoresist is applied to the surface of the thin filmsuch that an area of the thin film that is to be etched away is leftexposed. The photoresist is exposed using a light source andsubsequently developed. For example, the photoresist could be exposed tolight or cured using a laser to pattern the cured areas or anultraviolet (UV) lamp with a mask shading the areas that are notintended to be exposed to light. In an embodiment, the photoresist isexposed in a lithographic stepper or scanner. In an embodiment, aportion of the photoresist is removed.

In an embodiment, the photoresist is a positive photoresist. In apositive photoresist, a portion of the photoresist that is exposed tolight becomes soluble to a photoresist developer while the unexposedportion of the photoresist remain insoluble to the photoresistdeveloper.

In an embodiment, the photoresist is a negative photoresist. In anegative photoresist, a portion of the photoresist that is exposed tolight becomes insoluble to a photoresist developer while the unexposedportion of the photoresist is soluble to the photoresist developer.

In an embodiment, a photoresist layer is a positive tone photoresistmaterial and a positive tone development process removes the exposedportion of the photoresist layer. In an embodiment, the photoresistlayer is a negative tone photoresist material and a negative tonedevelopment process removes unexposed portions of the photoresist layer.In an embodiment, a photoresist developer is a liquid that dissolvesunexposed or uncured resin in a negative tone photoresist. In anembodiment, a photoresist developer is a liquid that dissolves exposedor cured resin in a positive tone photoresist. The resist developmentprocess results in of a portion of the sample surface with the thin filmand a portion of the sample surface coated with photoresist.Subsequently, the resist pattern is transferred to the thin film byetching. In an embodiment, a stripping process is performed after anetching to remove the remainder of the photoresist.

Etching of Superconductor

In an embodiment, the thin film is a superconducting material. In anembodiment, an etching process is used to remove a portion of thesuperconducting material. For example, material is removed such thatconductive lines are formed on the surface of the base material. In anembodiment, the portion of the superconducting material that is removedis defined by the photoresist left after development. In an embodiment,the bridge is defined by the etching process. In an embodiment, theetching is a dry etching process. For example, the etching gases couldcontain chlorine, chlorinated molecules, and/or fluorinated molecules.In an embodiment, the etching process includes etching, until a systemdetects the base material and then stops the etching process. In anembodiment, the etching process is anisotropic. In an embodiment, thebridge is 300 nm to 10 μm wide. In an embodiment, the length of anunderpass line under the bridge is approximately 2 to 30 times the widthof the bridge depending on the frequency and impedance requirements ofthe circuit. In an embodiment, the ratio of the width of the bridge tothe width of the underpass line is tailored to optimize the performancefor a desired application. In an embodiment, the underpass line is asignal line.

In an embodiment, the pattern is designed such that the photoresist maskon the bridge is formed with holes. In an embodiment, the smallestlateral dimension of the bridge is larger than the thickness of thefilm. In an embodiment, the pattern is designed such that the bridgewill be multiple substantially parallel bridges. For example, the bridgecould actually be two bridges with crossbeams connecting the twobridges. In an embodiment, the etching could form the bridge into a meshshape with multiple holes. In an embodiment, the first photo resistpattern defines a set of one or more holes, where the set of one or moreholes is sized such that the base material can be etched through the oneor more holes, and where the set of one or more holes is sized such thatmetal cannot be deposited through the set of one or more holes. In anembodiment, the first photo resist pattern defines a set of one or moreholes, where the set of one or more holes is sized such that metal canbe deposited through the set of one or more holes. In an embodiment, thedeposited metal is aluminum.

Underpass Photoresist Layer

In an embodiment, a photoresist is applied to control the etching of thebase material under the bridge. For example, a photoresist could beapplied, similar to before, to outline the area underneath the bridgewhere the base material is to be etched away, such that the basematerial in other parts is covered by photoresist during a base materialetching process.

In an embodiment, a photoresist is not applied to control the etching ofthe base material. For example, the base material (i.e., the basematerial is silicon) could be etched from substantially everywhere onthe surface that the superconducting material is exposed.

Etching Underpass

In an embodiment, the base material under the bridge is etched away torelease it from the substrate. Once released, the compressive stress inthe bridge buckles the bridge, away from the base material, to form anoverpass. In an embodiment, the etching of the base material (forexample, silicon) is an isotropic process. In an embodiment, thevertical depth of the etch is defined by the desired lateral etchrequired to release the bridge. In an embodiment, the depth could betailored to increase performance of a component. In an embodiment, thegap spacing between the bridge and the underpass is designed to permitsignificant transmission through the coplanar waveguide (CPW). In anembodiment, the gap is designed for minimizing the transmission loss. Inan embodiment, once the base material is sufficiently etched under thebridge, the compressive stress in the bridge will cause the bridge tobuckle up. In an embodiment, the gap is increased by controlling thelength of the bridge. For example, increasing the length of the bridgewill increase the gap.

Referring to FIG. 1A-1C, in an embodiment, a metal coating 120 isdeposited onto a base material 110. Metal coating 120 is deposited insuch a way such that metal coating 120 has a gradient of compressivestress across its thickness. A portion of metal coating 120 is removedforming channels 130 and 132, and leaving metal lines 140, 150, and 160.FIG. 1B is a cross section of a structure 100 at A1. FIG. 1C is a crosssection of structure 100 at B1. The width of metal line 140 and thewidth of metal lines 150 and 160 can be tailored depending therequirements of structure 100. In an embodiment, the first metal line,the second metal line and the third metal line are not indicative of anorder, but instead used as differentiation between different lines.

Referring to FIG. 1D-1F, in an embodiment, a portion of base material110 is removed from channels 130 and 132 and under metal line 140. Oncebase material 110 is removed from underneath metal line 140, metal line140 arcs out from structure 100, due to the compressive stresses acrossthe thickness of metal coating 120, to form a bridge 142. FIG. 1E is across section of structure 100 at A2. FIG. 1F is a cross section ofstructure 100 at B2. The resist mask that is used to pattern metalcoating 120 and/or base material 110 is not shown in FIG. 1A-1F. In anembodiment, the remaining resist is removed after completion of theetch. In an embodiment, there is a delamination between metal coating120 and base material 110 where bridge 142 meets the attached portion ofmetal coating 120.

Referring to FIG. 1G-1I, in an embodiment, a connecting metal line 170is deposited on metal lines 150, 160 and on base material 110, such thatmetal lines 150 and 160 are connected under bridge 142 withoutcontacting bridge 142. In an embodiment, the surface of metal coating120, including metal lines 150 and 160, are cleaned before deposition.In an embodiment, connecting metal line 170 is a continuous line. In anembodiment, metal line 170 is substantially the same width as metallines 150 and 160. In an embodiment, metal line 170 is slightly wider ornarrower than metal lines 150 and 160. FIG. 1H is a cross section ofstructure 100 at A3. In an embodiment, not shown, base material 110 willbe undercut by the etching process. For example, in FIG. 1I, basematerial 110 could be partially removed under metal coating 120 suchthat channel 130 would be wider than depicted in some areas. Likewise,in an embodiment, the etching process removes material such that theresulting channel structure has rounded walls and corners instead of thestraight edges and the sharp corners depicted (for example, straightwalls and sharp corners as shown by channel 130). FIG. 1I is a crosssection of structure 100 at B3. In an embodiment, a gap 180 iscontrolled based on the design of the bridge.

Sacrificial Material

In an embodiment, a sacrificial material is used underneath the bridgestructure. In an embodiment, the sacrificial material is asuperconducting material. In an embodiment, the sacrificial material isdeposited on the substrate before the deposition of the thin film. Forexample, a sacrificial superconducting material could be deposited onthe surface of the base material and a thin film (i.e., a multilayersuperconducting material) could be deposited on the surface of thesacrificial superconducting material. In an embodiment, the sacrificialsuperconducting material is 5-500 nm thick. In an embodiment, thesacrificial superconducting material and the multilayer superconductingmaterial are different materials. For example, the sacrificialsuperconducting material could be Titanium, Titanium Nitride, and/orAluminum, and the multilayer superconducting material could be Niobiumor Niobium Nitride.

In an embodiment, a photoresist is deposited and patterned, and a firstetching is performed. In an embodiment, in the first etching, only themultilayer superconducting material is substantially etched away in thearea where there is no photoresist pattern. In an embodiment, in thefirst etching, both the multilayer superconducting material and thesacrificial superconducting material are etched away in the area wherethere is no photoresist pattern. In an embodiment, a secondsuperconducting material etching is performed, where the etchant isspecifically designed to etch away the sacrificial superconductingmaterial (exposed by the first etching) and leave the majority of themultilayer superconducting material. For example, the first etchingcould define a metal line and a bridge structure. The second etchingcould undercut the sacrificial superconducting material from under thebridge structure, thereby releasing the bridge structure from thesuperconducting material.

Referring to FIG. 2A-2C, in an embodiment, a sacrificial layer 215 isfirst deposited on a base material 210 and a metal coating 220 is placedon sacrificial layer 215. In an embodiment, base material 210 issilicon, metal coating 220 is a superconducting material, andsacrificial layer 215 is a superconducting material. A portion of metalcoating 220 is removed forming channels 230 and 232, and leaving a metalline 240 and metal lines 250 and 260. FIG. 2B is a cross section of astructure 200 at C1. FIG. 2C is a cross section of structure 200 at D1.

Referring to FIG. 2D-2F, in an embodiment, a portion of sacrificiallayer 215 is removed from channels 230 and 232 and under metal line 240.Once sacrificial layer 215 is removed from underneath metal line 240,metal line 240 arcs out from base material 210, due to the compressivestresses across the thickness of metal coating 220, to form a bridge242. In an embodiment, once sacrificial layer 215 is removed basematerial 210 is etched away. In an embodiment, once sacrificial layer215 is removed base material 210 is not etched away. FIG. 2E is a crosssection of structure 200 at C2. FIG. 2F is a cross section of structure200 at D2. In an embodiment, sacrificial layer 215 is undercut duringthe etching process such that a portion of sacrificial layer 215 nearthe edge of a channel, such as channel 230, would be removed. In anembodiment, metal coating 220 and bridge 242 are substantially the samethickness. In an embodiment, there is a slight delamination betweenmetal coating 220 and sacrificial layer 215 where bridge 242 meets theattached portion of metal coating 220. In an embodiment, there is aslight delamination between sacrificial layer 215 and base material 210where bridge 242 meets the attached portion of metal coating 220. In anembodiment, the etching process would undercut metal coating 220 suchthat a portion of sacrificial layer 215 would be removed under metalcoating 220.

Referring to FIG. 2G-2I, in an embodiment, a connecting metal line 270is deposited on metal lines 250, 260 and on base material 210, such thatmetal lines 250 and 260 are connected under bridge 242 withoutconnecting metal line 270 contacting bridge 242. In an embodiment,connecting metal line 270 is also partially deposited on an exposedportion of sacrificial layer 215 under the edge of metal lines 250 and260. In an embodiment, metal line 270 is substantially the same width asmetal lines 250 and 260. In an embodiment, metal line 270 is slightlywider or narrower than metal lines 250 and 260. In an embodiment, thesurface of metal coating 220, including metal lines 250 and 260 iscleaned before deposition. In an embodiment, connecting metal line 270is a continuous line without any breaks. FIG. 2H is a cross section ofstructure 200 at C3. FIG. 2I is a cross section of structure 200 at D3.In an embodiment, a gap between connecting metal line 270 and bridge 242is controlled based on the design of the bridge. In an embodiment, theconnecting metal line is at least one metal line, and the at least onemetal line is aluminum.

Lift Off Resist

In an embodiment, a lift off process is used to form a metal linerunning under the bridge on the base material. A lift off layer isapplied to the surface of the substrate, then a photoresist layer isapplied to the surface of the lift off layer. The photoresist is curedand developed, as before, leaving a pattern in the photoresistcorresponding to a path for the metal line. In an embodiment, the liftoff resist, in the path for the metal line, is removed during thedevelopment step. For example, the lift off resist could beremoved/etched by the photoresist developer or a different materialand/or process. In an embodiment, the lift off resist is removed wherethe photoresist pattern is removed. For example, in an area where thephotoresist is removed, the uncovered lift off resist could be washedaway. There could be some undercutting of the lift off resist underneathof the photoresist during the development process. In an embodiment, theshadow of the bridge could cause the photoresist to not be cured underthe bridge. In an embodiment, an undercutting of the lift off resist,during the development process, will remove the lift off resistunderneath the photoresist under the bridge creating a continuous pathfor the metal line to be deposited into. In an embodiment, a metal lineis deposited underneath the bridge structure. For example, a resistprocess could be used to define a channel for the metal line to bedeposited in. In an embodiment, photolithography has an opticalproximity effect that allows development of an area of the resist underthe bridge that does not have line of sight to the source of thephotons. Unexposed resist underneath the bridge, if needed, can beremoved by overdeveloping the resist after the optical exposure. In anembodiment, a defined surface of the base material is the exposedsurface of the base material where the at least one metal line will notbe deposited, such that the area without resist will be where the atleast one metal line will be deposited. In an embodiment, the resist isformed on other surfaces (such as the surface of the metal coating) suchthat substantially all of a top surface of the entire formed structureis coated with the resist except where the at least one metal line willbe formed.

Electron-Beam Lithography

In an embodiment, electron-beam lithography (EBL) is used to expose theresist. In EBL, a beam of electrons exposes the portions of the resistthat are to be developed (for a positive tone resist). Scattering ofelectrons to areas nearby the exposed areas causes a proximity effect,where, for example, the resist under the bridge that does not have lineof sight to the source of the electrons can be exposed.

Aluminum Deposition

In an embodiment, aluminum is deposited on the surface of the substrateat a steep angle. In an embodiment, the surface of the superconductingmetal is ion milled to remove any built-up oxide layer before aluminumdeposition. In an embodiment, the aluminum for the underpass isdeposited in a lift off process.

Bridge Design

In an embodiment, the bridge can be designed with holes. In anembodiment, the bridge can be designed without holes. In an embodiment,a first set of holes will be large enough to allow etching of the basematerial or sacrificial superconducting metal under the bridge, but notlarge enough to allow aluminum deposition under the bridge where it isnot desired. In an embodiment, a first set of holes will be small enoughto allow etching of the base material or sacrificial superconductingmetal under the bridge, but not large enough to allow aluminumdeposition under the bridge. In an embodiment, a second set of one ormore holes in the bridge are large enough to allow aluminum depositionunder the bridge.

Referring to FIGS. 3A and 3B, in an embodiment, the bridge structure isformed with cutouts, such as holes 390 and a rectangle 395. For example,holes 390 and rectangle 395 can be removed from a metal coating 320during etching. In an embodiment, holes 390 are designed to allow a basematerial to be etched away, but not large enough for metal to bedeposited on the base material underneath the bridge when a connectingmetal line 370 is deposited. In an embodiment, connecting metal line 370is substantially the same width as metal lines 350 and 360. In anembodiment, rectangle 395 is positioned over the area where connectingmetal line 370 is to be deposited. For example, rectangle 395 can bedesigned to be large enough for the base material to be etched away andconnecting metal line 370 to be deposited. In an embodiment, connectingmetal line 370 will be substantially the same width as rectangle 395. Inan embodiment, connecting metal line 370 may be slightly wider ornarrower than rectangle 395.

Referring to FIG. 4A-4C, in an embodiment, a bridge structure is formedon a component 400 with multiple sets of holes 490 and rectangles 495,such that multiple areas under the bridge have base material etchedaway, and multiple areas can have connecting metal lines 471, 472, and473 deposited. For example, connecting metal line 471 could be depositedsuch that an area 452 is connected to an area 462 through connectingmetal line 471. For example, connecting metal line 472 could bedeposited such that a metal line 450 is connected to a metal line 460through connecting metal line 472. For example, connecting metal line471 could be deposited such that an area 454 is connected to an area 464through connecting metal line 473.

Buckling Inducement

In an embodiment, it may be necessary to apply external forces to causethe suspended structure (i.e., a bridge) to buckle outwards. Forexample, a voltage bias could be applied to the bridge and a groundstructure could be placed near the bridge to cause the bridge to buckleoutwards. In an embodiment, the bridge is pulled away from the basematerial. In an embodiment, the method of fabricating the bridgeincludes pulling the at least one bridge away from the base material. Inan embodiment, the bridge is at least one bridge. In an embodiment, thepulling comprises pulling the at least one bridge away from the basematerial by using electrostatic force. In an embodiment, the pulling isperformed using electrostatic force. In an embodiment, outwards meansaway from the base material.

Referring to FIG. 5, in an embodiment, a voltage bias is used tofacilitate buckling of a bridge structure 542. FIG. 5 is a cross sectionof a bridge structure according to an embodiment of the presentdisclosure. In an embodiment, a negative charge is applied to a groundstructure 545 and a positive charge is applied to bridge structure 542such that bridge structure 542 is induced to buckle away from a basematerial 510. In an embodiment, the voltage bias could be done beforedepositing a connecting metal line (such as metal lines 170, 270, and370). In an embodiment, the voltage bias could be done after depositinga connecting metal line (such as metal lines 170, 270, and 370). In anembodiment, the voltage bias could be used to facilitate buckling ofbridge structure 542 any time after the formation of bridge structure542.

Test Structure

Referring to FIG. 6, in an embodiment, a test structure is formed totest the bridge structure fabrication. In an embodiment, the teststructure has four contact pads. A first contact pad 610 is connected toa second contact pad 620 by a bridge 650, created according to anembodiment of the present disclosure. Second contact pad 620 isconnected to a third contact pad 630 by an underpass, created accordingto an embodiment of the present disclosure. For example, the underpasscould be comprised of metal lines 661, 662, and 660 formed from a metalcoating, and connecting metal lines 671 and 672 formed under bridges 650and 652. Connecting metal line 671 could connect metal line 661 to metalline 660 without directly touching bridge 650. For example, connectingmetal line 672 could connect metal line 662 to metal line 660 withoutdirectly touching bridge 652. Third contact pad 630 is connected to afourth contact pad 640 pad by bridge 652, created according to anembodiment of the present disclosure. In an embodiment, the bridge andunderpass structures can be tested by measuring the connectivity betweenthe various contact pads. For example, a conductivity between firstcontact pad 610 and second contact pad 620 signifies properly formedbridge 650. A complete connection between second contact pad 620 andthird contact pad 630 could signify properly formed metal lines 671 and672 in the underpass. A complete connection between third contact pad630 and fourth contact pad 640 could signify a properly formed secondbridge 652 between third contact pad 630 and fourth contact pad 640. Inan embodiment, the conductivity between each contact pad will have anexpected range of resistance. For example, the conductive path betweenfirst contact pad 610 and fourth contact pad 640 could have a differentresistance from the conductive path between first contact pad 610 andthird contact pad 630. In an embodiment, a variation of that expectedresistance signifies an improperly formed part.

Referring to FIG. 7, in an exemplary embodiment, a method 700 of thepresent invention is configured to perform an operation 710 ofdepositing a superconducting film on a surface of a base material,wherein the superconducting film is formed with a compressive stress,wherein the compressive stress is higher than a critical buckling stressof a defined structure, operation 720 of forming a first photoresistpattern on the superconducting film, wherein the photoresist patterndefines at least one bridge, operation 730 of etching an exposed area ofthe superconducting film, wherein the etching of the exposed area of thesuperconducting film creates the at least one bridge, operation 740 ofetching the base material, wherein the etching of the base materialforms a gap between the at least one bridge and the base material,operation 750 of forming a second photoresist pattern on the surface,wherein the second photoresist pattern defines a space for at least onemetal line, operation 760 of cleaning at least a part of the surface ofthe superconducting film, operation 770 of depositing the at least onemetal line on at least part of the superconducting film and at leastpart of the base material, wherein at least the metal line connects afirst part of the superconducting film and a second part of thesuperconducting film, wherein the at least one metal line runs under thebridge, and operation 780 of removing the second photoresist pattern.

Referring to FIG. 8, in an exemplary embodiment, a method 800 of presentinvention is configured to perform an operation 810 of depositing afirst superconducting film on a surface of a base material, operation820 of depositing a second superconducting film on a surface of thefirst superconducting film, wherein the second superconducting film isformed with a compressive stress, wherein the compressive stress ishigher than a critical buckling stress of a defined structure, operation830 of forming a first photoresist pattern on the second superconductingfilm, wherein the photoresist pattern defines at least one bridge,operation 840 of etching an exposed area of the second superconductingfilm, wherein the etching of the exposed area of the secondsuperconducting film creates the at least one bridge, operation 850 ofetching the first superconducting film, wherein the etching of the firstsuperconducting film forms a gap between the at least one bridge and thebase material, operation 860 of forming a second photoresist pattern onat least an exposed surface of the base material, wherein the secondphotoresist pattern defines a space for at least one metal line,operation 870 of cleaning at least a part of a surface of the secondsuperconducting film, operation 880 of depositing the at least one metalline on at least part of the second superconducting film and at leastpart of the base material, wherein at least the metal line connects afirst part of the second superconducting film and a second part of thesecond superconducting film, wherein the at least one metal line runsunder the bridge, and operation 890 of removing the second photoresistpattern.

It will be understood that when an element is described as being“connected,” “deposited on,” or “coupled” to or with another element, itcan be directly connected or coupled to the other element or, instead,one or more intervening elements may be present.

The descriptions of the various embodiments of the present disclosurehave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

“Present invention” does not create an absolute indication and/orimplication that the described subject matter is covered by the initialset of claims, as filed, by any as-amended set of claims drafted duringprosecution, and/or by the final set of claims allowed through patentprosecution and included in the issued patent. The term “presentinvention” is used to assist in indicating a portion or multipleportions of the disclosure that might possibly include an advancement ormultiple advancements over the state of the art. This understanding ofthe term “present invention” and the indications and/or implicationsthereof are tentative and provisional and are subject to change duringthe course of patent prosecution as relevant information is developedand as the claims may be amended.

“And/or” is the inclusive disjunction, also known as the logicaldisjunction and commonly known as the “inclusive or.” For example, thephrase “A, B, and/or C,” means that at least one of A or B or C is true;and “A, B, and/or C” is only false if each of A and B and C is false.

What is claimed is:
 1. A process comprising: depositing a superconducting film on a surface of a base material, wherein the superconducting film is formed with a compressive stress, wherein the compressive stress is higher than a critical buckling stress of a defined structure; forming a first photoresist pattern on the superconducting film, wherein the photoresist pattern defines at least one bridge; etching an exposed area of the superconducting film, thereby creating the at least one bridge; etching the base material, thereby forming a gap between the at least one bridge and the base material; forming a second photoresist pattern on at least a defined surface of the base material, wherein the second photoresist pattern defines a space for at least one metal line; cleaning at least a part of the surface of the superconducting film; depositing the at least one metal line on at least part of the superconducting film and at least part of the base material, wherein the at least one metal line connects a first part of the superconducting film and a second part of the superconducting film, wherein the at least one metal line runs under the bridge; and removing the second photoresist pattern.
 2. The process of claim 1 further comprising pulling the at least one bridge away from the base material.
 3. The process of claim 2, wherein the pulling comprises pulling the at least one bridge away from the base material by using electrostatic force.
 4. The process of claim 1, wherein the depositing the at least one metal line is performed by evaporation.
 5. The process of claim 1, wherein the first photo resist pattern defines a set of one or more holes, wherein the set of one or more holes is sized such that the base material can be etched through the one or more holes, and wherein the set of one or more holes is sized such that metal cannot be deposited through the set of one or more holes.
 6. The process of claim 1, wherein the first photo resist pattern defines a set of one or more holes, wherein the set of one or more holes is sized such that metal can be deposited through the set of one or more holes.
 7. The process of claim 1, wherein the base material is silicon.
 8. The process of claim 1, wherein the superconducting film comprises a material selected from the group consisting of aluminum and niobium.
 9. The process of claim 1, wherein the at least one metal line is aluminum.
 10. A process comprising: depositing a first superconducting film on a surface of a base material; depositing a second superconducting film on a surface of the first superconducting film, wherein the second superconducting film is formed with a compressive stress, wherein the compressive stress is higher than a critical buckling stress of a defined structure; forming a first photoresist pattern on the second superconducting film, wherein the photoresist pattern defines at least one bridge, etching an exposed area of the second superconducting film, thereby creating the at least one bridge; etching the first superconducting film, thereby forming a gap between the at least one bridge and the base material; forming a second photoresist pattern on at least a defined surface of the base material, wherein the second photoresist pattern defines a space for at least one metal line; cleaning at least a part of a surface of the second superconducting film; depositing the at least one metal line on at least part of the second superconducting film and at least part of the base material, wherein the at least one metal line connects a first part of the second superconducting film and a second part of the second superconducting film, wherein the at least one metal line runs under the bridge; and removing the second photoresist pattern.
 11. The process of claim 10 further comprising pulling the at least one bridge away from the base material.
 12. The process of claim 11, wherein the pulling comprises pulling the at least one bridge away from the base material by using electrostatic force.
 13. The process of claim 10, wherein the depositing the at least one metal line is performed by evaporation.
 14. The process of claim 10, wherein the first photo resist pattern defines a set of one or more holes, wherein the set of one or more holes is sized such that the base material can be etched through the one or more holes, and wherein the set of one or more holes is sized such that aluminum cannot be deposited through the set of one or more holes.
 15. The process of claim 10, wherein the first photo resist pattern defines a set of one or more holes, wherein the set of one or more holes is sized such that metal can be deposited through the set of one or more holes.
 16. The process of claim 10, wherein the base material is silicon.
 17. The process of claim 10, wherein first superconducting film comprises a material selected from the group consisting of aluminum and niobium. 